If you hold shift-click will deploy 5, ctrl-click will deploy 10.
You're correct the AI will do that sometimes based on their personality randomly chosen at the start of the game. Its something I picked up at the time when I reworked the AI but left it in.
Eventually when I make another game in this style I will add coalitions i.e. 1 power gets so strong compared to all the others, that all other factions declare war on the superpower at the same time.
I am glad you enjoy Saxon Kings, I am currently working on my next game which hopefully comes out this summer if I am not lazy.